Heat-to-electrical energy converter



June 6, 1956 c. w. HANSELL l 2,510,397

mwN-ELECTRICAL ENERGY cbm Filed DCT.. 2, 1945 INVENTOR CLARENCE W. HANELL ATTORNEY Patented June 6, 1950 C HEAT-TOELECTRCAL ENERGY l CONVERTER Clarence W. Hansell, Port Jeiferson, N. Y., as- -v signor to Radio Corporation of America, a corporation of Delaware Application October 2, 1946, Serial No. 700,644-

`13 Claims. (Cl. 1'11-330) This invention relates to a method of and apparatus for converting heat energy to electrical energy.

Briefly stated, the device of the invention for converting heat energy to electrical energy comprises two spaced surfaces having different 'electron work function potentials enclosing a gas or low pressure vapor with an ionizing potential intermediate the two workfunction potentials of the spaced surface. Putting it in other words, the low pressure vapor is confined between two dissimilar surfaces, and the ionizing potential of the -vapor is chosen to have a value intermediate the electron emission work functions of the two dissimilar surfaces. Vapor evaporates from one surface in the form of ions and from the other in the form of neutral molecules. In passing back and forth between the two dissimilar surfaces, due to condensation and re-evaporation, the vapor molecules transport electrons in only one direction, deriving energy from heat applied to maintain the temperature. The vapor pressure should be such that, at the operating temperature, the surface with the highest electron emission work function has less than enough absorbed vapor to form a layer one molecule thick.

The following definitions are given to aid in an understanding of the principles of the invention: A work function potential is the amount of energy expressed in volts required to separate an electron from a surface. An ionizing potential is the energy expressed in volts required to Fig. 4 illustrates several of the devices of Fig. l n

arranged in electrically parallel relation;

Fig'. 5 illustrates the device of the invention used to measure temperature; and

Fig. 6 illustrates a temperature controlled reservoir for maintaining a desired range of vapor pressure between the surfaces of the device of the invention.

In' the drawings the same parts are represented by the same reference numerals. f

Referring to Fig., 1, there is shown 'a pair of metallic surfaces A and B of dissimilar work functions enclosing therein a low pressure vapor C. Glass seals G are provided for conning the vapor C between the two surfaces A and B. In practice, the assemblage of A, B, C and G may take on the appearance of a wafer. Connected between surfaces A and B by means of leads I0 is a meter M.

As an illustration of thematerials which may be used in the device of Fig. 1, surface A may be tantalum, molybdenum, copper, mercury, tungsten, silver, iron, gold or nickel; while surface B may be zinc, or magnesium. The low pressure vapor C may be cesium, rubidium, potassium or sodium. An essential requirement in the selection of the materials for A, B and C is that surfaces A and B have work functions which are different and greater in the one case, and lower in the other case than the ionizing potential of the vapor. Surface A should have a work function greater than, while surface B should have a work function less than the ionizing potential of the vapor material C.' The table given below enables an easy selection of the elements which may be used in constructing the device of the invention.

The arrows shown in Fig. 1 indicate the direction of the inilow of heat energy which isto be converted to electrical energy, in turn measured by the meter M.

The vapor C should be a gas or vapor material which has a low boiling point and which is easily ionizable, in order to obtain a large current in moderate temperatures below the temperatures -of substantial thermionic electron emission.

This vapor material or gas C is conflned at low pressure between the surfaces A and B whose work functions should not be too much abeve and below the ionizing potential of the gas. The gas or vapor C preferably should be at a pressure such that the mean free pathv of molecules and ions in the vapor is very roughly equal to the spacing of the surfaces A and B. The surfaces also should be at as close a spacing asis practical without contact and without causing a short circuit, electrically.

For the invention to operate properly, the temperature range for a given condensable gas should be such that an extremely small amount less than one molecule thick is adsorbed on both surfaces. In other words, the evaporation must not be from thesurface of the condensed gas but should be from the base material constituting surfaces A and B. Otherwise, the evaporation molecules will not be ionized. Evidence exists that it is not necessary to have even a single monomolecular layer to make evaporation from the gas condensed on a Isurface predominate.

Fig. 2 is givento explain the principles of the invention. The ionized molecule leaving the surface A is labeled M and travels in the direction of surface B along the path shown by the arrows. As the ionized molecule M leaves surface A, it has a positive charge. When the molecule leaves surface B, it has gained an electron and has become neutral. This neutral molecule is represented by the letter N. The neutral lmolecule leaving surface B is evaporated and travels generally toward surface-A. It will thus be seen that the molecules are evaporated from both surfaces. The evaporation from surface A is anion while the evporation from surface B is a neutral molecule. In passing back and forth between surfaces A and B, the vapor molecules transport 'electrons in only one direction, due to condensation and re-evaporation, and derive energy from heat applied to maintain the temperature.

Experiments have shown that when molecules of an easily ionizable vapor such as cesium are vaporized from the surface of a metal having a large electron emission work function, such as platinum, then the evaporating' molecules will be almost all of them ionized. It is probable that ionization takes place because the molecules, before they leave the surface are bound to it by their valence electrons which have become a part of the surface.r When thermal agitation causes the molecules to break loose from the surface the valence electrons must pull loose either from the surface or from the molecules. quired for the electrons to pull away from the molecules is, of course, equal in volts to the ionization potential whereas the energy in volts required to pull the electron out of the surface is equal to the electron emission work function. The electrons will therefore go with the molecules when the ionization potential is greater than the electron emission work function but they will pull loose from the molecules and remain in the surface when the ionization potential is less than the electron emission work function.

The table for determining whether or not evaporation of one material from the surface of another is accompanied by ionization of the evaporated material is given below for many elements, although it should be understood that many compounds besides the 92 known elements may be used in constructing a successful energy converter. Ionization will take place if the ionizing potential of the evaporated material is lessy than the work function of the surface.

The energy relonizing Potential Volts .like a loi-metallic wafer.

Barium on oxide (oxido cathode) ses e ereewssmwmessr `Cesium Rubidium eaesaasesasssss a Potassium. 4. 33

Thodum Palladium 6. 3 Platinum l Oxygen film on tungsten.

In practice, in constructing the device of the invention, it is preferred that instead of the glass seal G the insulation between the surfaces A and B (which confine the gas therebetween) have a much longer surface path than the short path over the surface of the glass seals shown, in order to reduce leakage currents over the surface of the insulation caused by any material which might condense on the insulation. As an illustration, the ends of the metallic surfaces A and B may be bent outward to provide increased separation and the internal surfaces of the insulation such as G may be corrugated to increase the leakage path.

The complete assemblage of Fig- 1 may look These wafers can be stacked up in the electrically series or parallel relation depending upon whether it is desired to have a high voltage-low current, or a high current-low voltage output.

Fig. 3 shows how the device of. Fig'. 1 may be stacked up in electrically series relation to provide an outputr of relatively high voltage and low current. It should be noted that the intermediate surfaces A and B are in direct contact with each other. These two intermediate surfaces may oonstitute a bi-metallic wafer.

Fig. 4 shows how the device of Fig. 1 can be stacked up in electrically parallel relation to pro- 8 faces A and B are connectedvia lead I8 and resistor -I8 to opposite terminals of `a battery 20. Across the resistor I8 or connected to suitably spaced points on this resistor, is a box 22 whi'ch may represent a meter, an amplifier, or a relay. Output is obtained from across the resistor I8 and supplied via leads 24 to the box 22.

, Broadly, it will be noted, the arrangement of Fig. is one in which the electrical power which can be controlled is -very much greater than the electrical power which can be generated by the device itself.. The device simply serves as an electrical resistance, the value of which varies with variation in temperature. t

In practice it may in some cases be desirable to provide a control for the-vapor pressure of the ionizable gas between the two dissimilar plates. This may be accomplished by joining to the space between the plates a reservoir or volume which can be held at any desired temperature, less than the temperature of the dissimilar surfaces. Extra vaporizable material may be kept stored in this reservoir which may be released for use by raising the reservoir temperature. Likewise, anyv excess of vapor may be reduced by lowering the reservoir temperature to cause condensation and lowering of the vapor pressure. One arrangement forachieving these results is shown in Fig. 6 which shows a construction like that of Fig. 1 modified by joining to the space C between the plates A and B a reservoir R which is held at any desired temperature less than the temperature of the dissimilar surfaces by'v means of a heater coil H, in turn supplied with suitable heater power supply over leads L. The temperature controlled reservoir includes a thermostat T coupled to the reservoir wall.

In workinggtwoinmercial devices based on the invention it is anticipated that problems will arise due to chemical reactions, or amalgamation, between the vaporizable material and the surfaces or the insulation. Such reactions will limit the useful life of the devices and, as is usually the case, considerations of life as well as of initial performance, operating temperature range, etc. will ,1,requiraproperbalancing,for best overall results. tf'is :anticipated that'development of devices based on the invention, and research to reveal the ionizing potentials, vapor pressures and chemical inertness of various possible vapors, may very well lead to results far superior to those which may be obtained initially by using only the elements.

What is claimed is:

1. A heat energy to electrical direct current energy converter comprising two spaced surfaces having different electron work function potentials and an ionizable gas or vapor confined between them, the ionizing potential of the gas or vapor being intermediate said two work function potentials, the surfaces being held at such a temperature that evaporation of gas or vapor molecules occurs from the surfaces themselves and said molecules bound back and forth between the surfaces therebytransferring electrons from the surface of lower work function potential to the surface of higher work function potential.

2. An energy converter comprising two substantially parallel plates spaced apart at their edges by insulating material, said plates having different work functions, and an' ionizable material i is less than the ionizing potential of said ionizable material. f

3. An energy converter comprising two spaced surfaces having different electron work function potentials and an ionizable gas or vapor confined between them, the ionizing potential of the gas or vapor being intermediate said two work function potentials, and a meter connected'between said two surfaces.

4. An energy converter comprising two spaced surfaces having different electron work function potentials and an ionizable gas or vapor confined between them, the ionizing potential of the gas or vapor -being intermediate said two work function potentials, and av source of unidirectional potential having one terminal connected to one surface and the other terminal connected to the other surface through a resistor, and a utilization circuit coupled across said resistor.

5. A device for converting heat energy to electrical energy, comprising a, pair of spaced metallic plates having different work function potentials, and means for confining between said plates A an ionizable gaseous material having an ionizing potential which is intermediate said two work ,than while the other plate has a work function which is less than the ionizing potential of said ionizable material, a source of electrical energy connected between said plates by means of leads, and a utilization circuit coupled to one of the leads connecting said source to said plates.

7. A temperature sensitive device comprising a pair of spaced metallic plates having different work function potentials, and means for confining between said plates an ionizable gaseous material having an ionizing potential which is intermediate said two work function potentials, a source of unidirectional potential having its negative terminal directly connected to the plate of greater work function and its positive terminal connected through a direct current impedance to the plate of lower work function, and a utilization circuit connected across spaced points on said direct current impedance.

8. A converter of heat energy into electrical energy comprising a plate selected from a group including zinc and magnesium, another plate spaced from said rst plate and selected from a group including tantalum, molybdenum, copper, mercury, tungsten, silver, gold, iron, and nickel, and an ionizablematerial confined between said plates and selected fromv a grou-p including cesium, rubidium, potassium and sodium.

9. An energy converter comprising a plate selected from a group including zinc and magne` sium, another plate spaced from said iirst plate and selected from a group including tantalum, molybdenum, copper, mercury, tungsten, silver, iron, gold, and nickel, and an ionizable gas or vapor material confined between said plates and selected from a group including cesium and rubidium, said ionizable material being at a pressure such that the mean free path of the ions in the ionizable material is very roughly equal to the spacing between said plates.

l0. 'I'he method of converting heat energy to electrical energy which comprises confining an ionizable gaseous material between two spaced surfaces whose work function potentials are on opposite sides of the ionizing potential of the gaseous material, and causing molecules of the gas to bound back and forth between the two surfaces by maintaining the surfaces at temperatures high enough to prevent substantial condensation of gas molecules upon them.

11. An energy converterwcomprising an assemblage of two spaced surfaces having diiferent electron work function potentials and connning between them an ionizableA material whose ionizing potential is intermediate the two work function potentials of said spaced surfaces, another simllar assemblage, and means for directly connecting thev plates of said two assemblages in electrically parallel relation to provide an output of low voltage and high current.

12. An energy converter comprising an assemblage of two spaced surfaces having different electron work function potentials Aand confining between them an ionizable material whose ionizing potential is intermediate the two work function .series relation to provide anfoutput of high voltage and low current.

8 13. A device electrically responsive to change in temperature comprising an ionizable vapor of a low ionizing potential confined between two surfaces, one having an electron emission work REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS p Number Name Date 1,244,217 Langmuir Oct. 23, 1917 p 1,795,730 Marsden Mar. 10, 1931 2,056,665 Frech Oct. 6. 1936 OTHER REFERENCES Thermionic Effects Caused by Vapors of Alkali Metals by Langmuir l: Kingdom, page 61' of Reprint from Proceedings of Royal Society, A, vol. 107, 1925. 

